The present invention relates generally to the field of Selective Catalyst Reactor (SCR) temperature control and in particular to a system and method for maintaining the combustion or flue gas entering the SCR system at or above the optimal catalytic reaction temperature, even when operating the boiler at reduced loads.
In operating a boiler with a Selective Catalyst Reactor (SCR) system, the effectiveness of the SCR is dependant upon the flue gas temperature entering the catalyst reactor. Most can operate within a temperature range of about 450 degrees F. to about 840 degrees F. Optimum performance may typically occur between about 570 degrees F. to about 750 degrees F. Typically, the desired gas temperature entering the SCR is about 580 degrees F. or greater. At a temperature of about 580 degrees F., the reaction of ammonia with NOx is optimized and the amount of the ammonia needed for the catalytic reaction is minimized. Therefore, for economic reasons the desired gas temperature entering the catalyst reactor should be maintained within the optimum temperature range of about 570 degrees F. to about 750 degrees F. at all loads.
However, as boiler load varies, the boiler exit gas temperature will drop below the optimal temperature of about 580 degrees F. To increase the gas temperature to about 580 degrees F., current practice has been to use an economizer gas bypass. The economizer gas bypass is used to mix the hotter gases upstream of the economizer with the cooler gas that leaves the economizer. By controlling the amount gas through the bypass system, a boiler exit flue gas temperature of about 580 degrees F. can be maintained at lower boiler loads.
With this approach, static mixing devices, pressure reducing vanes/plates and thermal mixing devices are required to make the different temperature flue gases mix before the gas mixture reaches the inlet of the catalytic reactor. In most applications, obtaining the strict mixing requirements for flow, temperature and the mixing of the ammonia before the catalyst reactor is often difficult.
In another approach to dealing with decreasing flue gas temperature entering a SCR reactor at reduced boiler loads, an economizer was fitted with a feedwater bypass to partially divert the feedwater away from the economizer in order to maintain the flue gas temperature.
Additional details of SCR systems for NOx removal are provided in Chapter 34 of Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright® 2005, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein. Flue gas temperature control using conventional economizers are described in U.S. Pat. No. 7,021,248 to McNertney, Jr. et al. and U.S. Pat. No. 6,609,483 to Albrecht et al., the texts of which are hereby incorporated by reference as though fully set forth herein.